Recently, there has been an increasing interest in energy storage technology. Electrochemical devices have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them.
In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy.
Many secondary batteries are currently available. Among these, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni—Cd, and H2SO4—Pb batteries.
The lithium secondary batteries consist of a cathode, an anode, a non-aqueous electrolyte solution and a separator. Among these, the separator should have the requirements so that while an electric insulator separates the cathode and the anode, the permeability of lithium ions is increased with the basis of high porosity which will eventually enhance ionic conductivity. As such a separator, a substrate made of a polyolefin-based polymer such as polyethylene have generally been used as it is favorable in terms of easy pore-formation, good chemical-resistance, mechanical property and thermal stability, as well as cheap price.
The preparation of a microporous separator from polyethylene has been widely conducted by a wet process which comprises mixing polyethylene with a diluent acting as a pore-forming agent, extruding the mixture, stretching the extruded resultant and extracting the diluent to prepare a microporous separator. However, such a wet process is not easy to suitably control the pore size of a porous membrane and is limited to sufficiently improving the high-temperature stability and good mechanical property of the polymer separator.